1. Field of the Invention
The present invention relates to an electronic scanning radar apparatus and a received wave direction estimating method, which can detect a target using a reflected wave from the target in response to a transmitted wave and which can be suitably used for a vehicle, and a received wave direction estimating program used therein.
Priority is claimed on Japanese Patent Application No. 2010-151046, filed Jul. 1, 2010, the content of which is incorporated herein by reference.
2. Description of Related Art
Electronic scanning type radars such as an FMCW (Frequency Modulated Continuous Wave) radar, a multi-frequency CW (Continuous Wave) radar, and a pulse radar have been known for some time.
In the radars, an arrival wave direction estimating method of an array antenna is used as a technique of detecting an arrival wave (or a received wave) from a target (a reflecting object).
As such an arrival wave direction estimating method, high-resolution (high-precision) algorithms such as an AR spectrum estimating method (FIG. 44) which can obtain high resolution without increasing the number of channels of a receiving antenna and a MUSIC (Multiple Signal Classification) method have been used. Such methods are described in Japanese Patent Application Nos. JP-A-2006-275840, JP-A-2009-156582, JP-A-2006-47282, JP-A-2009-25195, and JP-A-2009-162688, and also described in “Fundamentals of Digital Signals on MATLAB Multimedia Signal Processing,” written by Ikehara and Shimamura, published by Baifukan Co., Ltd., 2004, and “Adaptive Antenna Technology,” written by Kikuma, published by Ohmsha Ltd., 2003. The AR spectrum estimating method may be called a maximum entropy method (MEM) or a linear prediction method.
When an arrival wave direction from a target (a reflecting object) is estimated using such algorithms, the estimation process is performed after input data (data in which a noise component is mixed into a complex sinusoidal wave) expressed by a complex number is transformed into a matrix format of a correlation matrix.
Japanese Patent Application No. JP-A-2009-156582 describes an arrival wave direction estimating method used in an in-vehicle radar, in which a correlation matrix in a past control cycle is stored and is subjected to an averaging process (or an adding process) with a correlation matrix in the present control cycle and then a direction estimating process is performed, to suppress a noise component and to improve estimation precision.
As the number of channels of an array antenna becomes smaller, a radar apparatus becomes smaller in size and cost to be suitable for a vehicle, but the information quantity of data used for the estimation becomes more insufficient. Accordingly, even when an high-resolution algorithm is used, the estimation precision is lowered. That is, when the information quantity of data used for the estimation is insufficient, a received signal cannot be equivalently processed as an ideal sinusoidal wave, which affects the correlation process result. Accordingly, by performing an averaging process on the correlation matrix, it is possible to enhance the detection precision.
In such an algorithm, the estimation should be performed after an appropriate number of arrival waves (received waves) (model order in the case of the AR spectrum estimating method) is set. It is necessary to set an appropriate value when it is applied to an in-vehicle radar. For example, in the techniques described in Japanese Patent Application Nos. JP-A-2006-47282, JP-A-2009-25195, JP-A-2009-162688, and JP-A-2006-153579, since the algorithm such as the MUSIC method of estimating a received wave direction through the use of eigenvalue resolution is an algorithm in which a correlation matrix should be resolved in eigenvalues, the number of received waves is estimated by separating a signal component and a noise component on the basis of the magnitude relation of eigenvalues every time.
The AR spectrum estimating method is an algorithm having a relatively small computational load and can perform an azimuth estimating process on targets existing in a distance direction, which is a method suitable for use in in-vehicle radar. Compared with the MUSIC method, there is an advantage that the estimation can be performed without being sensitive to the setting of the number of received waves (=model order), and the higher model order tends to enhance the estimation precision.
However, when the number of received waves is much smaller than the set order and the noise component is great, there is a problem in that spurious peaks may appear.
“Adaptive Antenna Technology”, written by Kikuma, published by Ohmsha Ltd., 2003 discloses a method of estimating the number of received waves using an AIC (Akaike Information Criterion) method, an MDL (Minimum Description Length) method, or the like. However, this does not guarantee the high-precision estimation result. Studies of the smaller number of channels and the lower order suitable for in-vehicle radar have hardly been reported.